1. Field
The present disclosure relates to a photoelectric conversion element.
2. Description of the Related Art
A great deal of research and development has been carried out on solar cells for the purpose of increasing the photoelectric conversion efficiency using light in a wider wavelength range. For example, Physical Review Letters, vol. 97, p. 247701, 2006 (hereinafter referred to as the “non-patent document”) proposes a solar cell that can photoexcite electrons in two steps to use light with a long wavelength in such a manner that a superlattice miniband is formed between the valence band and conduction band of a base material.
The solar cell has quantum dots as disclosed in the non-patent document and has a structure in which a quantum dot layer having quantum dots is inserted in a compound solar cell. Such a structure enables the absorption of light in an unused wavelength range (the absorption of a photon with an energy less than the band gap of a base material) by two-step photoexcitation through quantum levels and enables a photocurrent to be increased. In the case where a superlattice miniband is formed by electronic bonding between quantum dots, carriers generated in the quantum dots migrate in the superlattice miniband to p-type and n-type base semiconductor regions by photoexcitation and are extracted outside.
At present, in a solar cell including a quantum dot layer, the extraction efficiency of carriers generated in the quantum dot layer is very low and therefore the photoelectric conversion efficiency is low. A cause of this is probably the fact that the efficiency of two-step light absorption through quantum levels (including a superlattice miniband) is low. In particular, in two-step light absorption, an absorption band from a quantum level corresponding to the second light absorption to a conduction band is narrower than an absorption band from a valence band corresponding to the first light absorption to a quantum level and the match between the second light absorption band and the spectrum of sunlight is low. Therefore, there is a problem in that the second light absorption is insufficient, although the first light absorption is sufficient.
Research and development has been also carried out to increase the efficiency of solar cells by the use of wavelength conversion materials. Japanese Unexamined Patent Application Publication No. 2014-22499 (hereinafter referred to as the “patent document”) describes that the wavelength range of light capable of being photoelectrically converted is expanded in such a manner that light in a wavelength region between two light absorption peaks causing transmission loss is wavelength-converted into light with a light absorption peak on the longer wavelength side, whereby the increase of the photoelectric conversion efficiency is attempted. However, the patent document does not disclose that incident light is converted into light with a wavelength corresponding to an optical transition from a quantum level corresponding to the second light absorption to a conduction band using a wavelength conversion material in order to increase the photoelectric conversion efficiency.